History of Pervious Concrete
How is it that concrete, a rock-solid building material, can be embraced as environmentally friendly? Thanks to developments in the concrete business over the last 30 years, concrete can now be made pervious (opposite of the impervious surface that it was long designed to be). As long as a century ago, designers in Europe recognized the value of porous concrete as structural insulation for buildings. And about 80 years ago in Europe, pervious concrete was starting to be used for roads as well. It's only in the last 20 years that porous concrete has gained a foothold for general construction purposes in the United States.
What is pervious concrete?
Pervious concrete is pretty much the same as regular concrete with little or no sand included as a binding agent that would normally fill in the gaps of the aggregate or stone that was part of the concrete recipe. Without the sand, small pockets develop between the stone and mix, allowing water to seep through the material after it sets (cures). The imagery usually associated with porous concrete is that of a Rice Krispie treat. You have the rice and marshmallow with little gaps in between. Keep that picture in mind.
Why is pervious concrete better than impervious concrete?
The answer depends on its use. And some of the questions have yet to be satisfactorily answered. Imagine you want to build a shopping mall with a large parking lot. Under most current building codes, when you add the footprint of the buildings and the parking lot, you typically have created a large impervious surface, so where does all the water go when it rains? It runs off the impervious surface and, in heavy rain, can cause flooding and other hazardous conditions. To address the runoff, most zoning regulations in most states require that there be no more water runoff after the development than it did before the development. The conventional result has been to build retention ponds where, through shrewd engineering, swales and grading, the water is directed to the holding ponds. It is held in the pond and then released in incremental amounts to avoid the negative effects of huge amounts of water running off the property all at once. If the lot is made with pervious concrete, the water seeps through the parking area into a base of aggregate and a layer of sand at the bottom. It eliminates ponding, eliminates retention ponds and preserves more land as open space.
How is it eco-friendly?
It is good for the environment in several ways. One need look no further than the University of New Hampshire in Durham where, in a public-private venture, the college and local concrete manufacturers and advocacy groups joined together to replace an existing parking lot with pervious concrete. Not only does it rain in New Hampshire but it snows (a lot) as well. There are two major benefits to the environment. One, the water percolates through the concrete, through the aggregate and sand below, which acts as a natural filter eliminating chemicals used in winter to melt the snow and oil as well as other chemicals emitted naturally by cars that park on the lot. The "green" parking lot is expected to help protect the nearby Oyster River. The college and other development has come under fire from residents in the past for overburdening the river with runoff and contaminants. Lastly, because the surface is porous, the traction for vehicles remains the same in wet and dry weather. And although the surface is a little bumpier than normal concrete, there is no ponding of water, which creates ice and requires more chemicals to keep the parking lot drivable. And despite the bumps, it passed the "eggs in a cart" test. No, really, they pushed an open cart of eggs across the lot to see if they would break. They didn't.
What are the drawbacks?
With all the positive things touted about pervious concrete, there must be some downsides or one would think it would be more widely used. Questions remain about its durability in colder climates. Normal concrete is subject to cracking through the normal thaw and freeze cycle that occurs in colder areas of the country. And while some pervious concrete roads in the South remain in use after 30 years, it hasn't passed the test of time in, say, Minnesota. At least one doctoral dissertation tested pervious concrete in laboratory conditions at Brigham Young University and showed promise, but governments (local ones in particular) aren't readily open to using taxpayer money to test the durability of pervious concrete over many winters. Also, the study showed that the holes that make the concrete porous can clog over time. Also, the slope of the land can't exceed 1 percent to allow the water to seep into the ground.
What are the positives?
Currently, the cost per cubic yard is slightly higher than run-of-the mill concrete. It is also a little more labor intensive. However, for larger projects, engineering costs can be reduced and so can the cost of land acquisition without the requirement for retention ponds. Insurance costs are also cut because retention ponds are legally considered an "attractive nuisance," meaning that if kids decide to play in the water, no matter if it's fenced, there are liability concerns. And because the concrete is a lighter color, it reflects heat, as opposed to black macadam, which absorbs the heat. The result is that the thermal increase in cities would be reduced with porous concrete.
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